Computer Networks 4th Ed Andrew S. Tanenbaum [Electronic resources] نسخه متنی

4.5 Broadband Wireless

We have been indoors too long. Let us now go outside and see if any interesting networking is going on there. It turns out that quite a bit is going on there, and some of it has to do with the so-called last mile. With the deregulation of the telephone system in many countries, competitors to the entrenched telephone company are now often allowed to offer local voice and high-speed Internet service. There is certainly plenty of demand. The problem is that running fiber, coax, or even category 5 twisted pair to millions of homes and businesses is prohibitively expensive. What is a competitor to do?

The answer is broadband wireless. Erecting a big antenna on a hill just outside of town and installing antennas directed at it on customers' roofs is much easier and cheaper than digging trenches and stringing cables. Thus, competing telecommunication companies have a great interest in providing a multimegabit wireless communication service for voice, Internet, movies on demand, etc. As we saw in Fig. 2-30, LMDS was invented for this purpose. However, until recently, every carrier devised its own system. This lack of standards meant that hardware and software could not be mass produced, which kept prices high and acceptance low.

Many people in the industry realized that having a broadband wireless standard was the key element missing, so IEEE was asked to form a committee composed of people from key companies and academia to draw up the standard. The next number available in the 802 numbering space was 802.16, so the standard got this number. Work was started in July 1999, and the final standard was approved in April 2002. Officially the standard is called ''Air Interface for Fixed Broadband Wireless Access Systems.'' However, some people prefer to call it a wireless MAN (Metropolitan Area Network) or a wireless local loop. We regard all these terms as interchangeable.

Like some of the other 802 standards, 802.16 was heavily influenced by the OSI model, including the (sub)layers, terminology, service primitives, and more. Unfortunately, also like OSI, it is fairly complicated. In the following sections we will give a brief description of some of the highlights of 802.16, but this treatment is far from complete and leaves out many details. For additional information about broadband wireless in general, see (Bolcskei et al., 2001; and Webb, 2001). For information about 802.16 in particular, see (Eklund et al., 2002).

4.5.1 Comparison of 802.11 with 802.16

At this point you may be thinking: Why devise a new standard? Why not just use 802.11? There are some very good reasons for not using 802.11, primarily because 802.11 and 802.16 solve different problems. Before getting into the technology of 802.16, it is probably worthwhile saying a few words about why a new standard is needed at all.

The environments in which 802.11 and 802.16 operate are similar in some ways, primarily in that they were designed to provide high-bandwidth wireless communications. But they also differ in some major ways. To start with, 802.16 provides service to buildings, and buildings are not mobile. They do not migrate from cell to cell often. Much of 802.11 deals with mobility, and none of that is relevant here. Next, buildings can have more than one computer in them, a complication that does not occur when the end station is a single notebook computer. Because building owners are generally willing to spend much more money for communication gear than are notebook owners, better radios are available. This difference means that 802.16 can use full-duplex communication, something 802.11 avoids to keep the cost of the radios low.

Because 802.16 runs over part of a city, the distances involved can be several kilometers, which means that the perceived power at the base station can vary widely from station to station. This variation affects the signal-to-noise ratio, which, in, turn, dictates multiple modulation schemes. Also, open communication over a city means that security and privacy are essential and mandatory.

Furthermore, each cell is likely to have many more users than will a typical 802.11 cell, and these users are expected to use more bandwidth than will a typical 802.11 user. After all it is rare for a company to invite 50 employees to show up in a room with their laptops to see if they can saturate the 802.11 wireless network by watching 50 separate movies at once. For this reason, more spectrum is needed than the ISM bands can provide, forcing 802.16 to operate in the much higher 10-to-66 GHz frequency range, the only place unused spectrum is still available.

But these millimeter waves have different physical properties than the longer waves in the ISM bands, which in turn requires a completely different physical layer. One property that millimeter waves have is that they are strongly absorbed by water (especially rain, but to some extent also by snow, hail, and with a bit of bad luck, heavy fog). Consequently, error handling is more important than in an indoor environment. Millimeter waves can be focused into directional beams (802.11 is omnidirectional), so choices made in 802.11 relating to multipath propagation are moot here.

Another issue is quality of service. While 802.11 provides some support for real-time traffic (using PCF mode), it was not really designed for telephony and heavy-duty multimedia usage. In contrast, 802.16 is expected to support these applications completely because it is intended for residential as well as business use.

In short, 802.11 was designed to be mobile Ethernet, whereas 802.16 was designed to be wireless, but stationary, cable television. These differences are so big that the resulting standards are very different as they try to optimize different things.

A very brief comparison with the cellular phone system is also worthwhile. With mobile phones, we are talking about narrow-band, voice-oriented, low-powered, mobile stations that communicate using medium-length microwaves. Nobody watches high-resolution, two-hour movies on GSM mobile phones (yet). Even UMTS has little hope of changing this situation. In short, the wireless MAN world is far more demanding than is the mobile phone world, so a completely different system is needed. Whether 802.16 could be used for mobile devices in the future is an interesting question. It was not optimized for them, but the possibility is there. For the moment it is focused on fixed wireless.

4.5.2 The 802.16 Protocol Stack

The 802.16 protocol stack is illustrated in Fig. 4-31. The general structure is similar to that of the other 802 networks, but with more sublayers. The bottom sublayer deals with transmission. Traditional narrow-band radio is used with conventional modulation schemes. Above the physical transmission layer comes a convergence sublayer to hide the different technologies from the data link layer. Actually, 802.11 has something like this too, only the committee chose not to formalize it with an OSI-type name.

Figure 4-31. The 802.16 protocol stack.

Although we have not shown them in the figure, work is already underway to add two new physical layer protocols. The 802.16a standard will support OFDM in the 2-to-11 GHz frequency range. The 802.16b standard will operate in the 5-GHz ISM band. Both of these are attempts to move closer to 802.11.

The data link layer consists of three sublayers. The bottom one deals with privacy and security, which is far more crucial for public outdoor networks than for private indoor networks. It manages encryption, decryption, and key management.

Next comes the MAC sublayer common part. This is where the main protocols, such as channel management, are located. The model is that the base station controls the system. It can schedule the downstream (i.e., base to subscriber) channels very efficiently and plays a major role in managing the upstream (i.e., subscriber to base) channels as well. An unusual feature of the MAC sublayer is that, unlike those of the other 802 networks, it is completely connection oriented, in order to provide quality-of-service guarantees for telephony and multimedia communication.

The service-specific convergence sublayer takes the place of the logical link sublayer in the other 802 protocols. Its function is to interface to the network layer. A complication here is that 802.16 was designed to integrate seamlessly with both datagram protocols (e.g., PPP, IP, and Ethernet) and ATM. The problem is that packet protocols are connectionless and ATM is connection oriented. This means that every ATM connection has to map onto an 802.16 connection, in principle a straightforward matter. But onto which 802.16 connection should an incoming IP packet be mapped? That problem is dealt with in this sublayer.

4.5.3 The 802.16 Physical Layer

As mentioned above, broadband wireless needs a lot of spectrum, and the only place to find it is in the 10-to-66 GHz range. These millimeter waves have an interesting property that longer microwaves do not: they travel in straight lines, unlike sound but similar to light. As a consequence, the base station can have multiple antennas, each pointing at a different sector of the surrounding terrain, as shown in Fig. 4-32. Each sector has its own users and is fairly independent of the adjoining ones, something not true of cellular radio, which is omnidirectional.

Figure 4-32. The 802.16 transmission environment.

Because signal strength in the millimeter band falls off sharply with distance from the base station, the signal-to-noise ratio also drops with distance from the base station. For this reason, 802.16 employs three different modulation schemes, depending on how far the subscriber station is from the base station. For close-in subscribers, QAM-64 is used, with 6 bits/baud. For medium-distance subscribers, QAM-16 is used, with 4 bits/baud. For distant subscribers, QPSK is used, with 2 bits/baud. For example, for a typical value of 25 MHz worth of spectrum, QAM-64 gives 150 Mbps, QAM-16 gives 100 Mbps, and QPSK gives 50 Mbps. In other words, the farther the subscriber is from the base station, the lower the data rate (similar to what we saw with ADSL in Fig. 2-27). The constellation diagrams for these three modulation techniques were shown in Fig. 2-25.

Given the goal of producing a broadband system, and subject to the above physical constraints, the 802.16 designers worked hard to use the available spectrum efficiently. One thing they did not like was the way GSM and DAMPS work. Both of those use different but equal frequency bands for upstream and downstream traffic. For voice, traffic is probably symmetric for the most part, but for Internet access, there is often more downstream traffic than upstream traffic. Consequently, 802.16 provides a more flexible way to allocate the bandwidth. Two schemes are used, FDD (Frequency Division Duplexing) and TDD (Time Division Duplexing). The latter is illustrated in Fig. 4-33. Here the base station periodically sends out frames. Each frame contains time slots. The first ones are for downstream traffic. Then comes a guard time used by the stations to switch direction. Finally, we have slots for upstream traffic. The number of time slots devoted to each direction can be changed dynamically to match the bandwidth in each direction to the traffic.

Figure 4-33. Frames and time slots for time division duplexing.

Downstream traffic is mapped onto time slots by the base station. The base station is completely in control for this direction. Upstream traffic is more complex and depends on the quality of service required. We will come to slot allocation when we discuss the MAC sublayer below.

Another interesting feature of the physical layer is its ability to pack multiple MAC frames back-to back in a single physical transmission. The feature enhances spectral efficiency by reducing the number of preambles and physical layer headers needed.

Also noteworthy is the use of Hamming codes to do forward error correction in the physical layer. Nearly all other networks simply rely on checksums to detect errors and request retransmission when frames are received in error. But in the wide area broadband environment, so many transmission errors are expected that error correction is employed in the physical layer, in addition to checksums in the higher layers. The net effect of the error correction is to make the channel look better than it really is (in the same way that CD-ROMs appear to be very reliable, but only because more than half the total bits are devoted to error correction in the physical layer).

4.5.4 The 802.16 MAC Sublayer Protocol

The data link layer is divided into three sublayers, as we saw in Chap. 8, it is difficult to explain now how the security sublayer works. Suffice it to say that encryption is used to keep secret all data transmitted. Only the frame payloads are encrypted; the headers are not. This property means that a snooper can see who is talking to whom but cannot tell what they are saying to each other.

If you already know something about cryptography, here comes a one-paragraph explanation of the security sublayer. If you know nothing about cryptography, you are not likely to find the next paragraph terribly enlightening (but you might consider rereading it after finishing Chap. 8).

At the time a subscriber connects to a base station, they perform mutual authentication with RSA public-key cryptography using X.509 certificates. The payloads themselves are encrypted using a symmetric-key system, either DES with cipher block chaining or triple DES with two keys. AES (Rijndael) is likely to be added soon. Integrity checking uses SHA-1. Now that was not so bad, was it?

Let us now look at the MAC sublayer common part. MAC frames occupy an integral number of physical layer time slots. Each frame is composed of sub-frames, the first two of which are the downstream and upstream maps. These maps tell what is in which time slot and which time slots are free. The downstream map also contains various system parameters to inform new stations as they come on-line.

The downstream channel is fairly straightforward. The base station simply decides what to put in which subframe. The upstream channel is more complicated since there are competing uncoordinated subscribers that need access to it. Its allocation is tied closely to the quality-of-service issue. Four classes of service are defined as follows:

Constant bit rate service.

Real-time variable bit rate service.

Non-real-time variable bit rate service.

Best-efforts service.

All service in 802.16 is connection-oriented, and each connection gets one of the above classes of service, determined when the connection is set up. This design is very different from that of 802.11 or Ethernet, which have no connections in the MAC sublayer.

Constant bit rate service is intended for transmitting uncompressed voice such as on a T1 channel. This service needs to send a predetermined amount of data at predetermined time intervals. It is accommodated by dedicating certain time slots to each connection of this type. Once the bandwidth has been allocated, the time slots are available automatically, without the need to ask for each one.

Real-time variable bit rate service is for compressed multimedia and other soft real-time applications in which the amount of bandwidth needed each instant may vary. It is accommodated by the base station polling the subscriber at a fixed interval to ask how much bandwidth is needed this time.

Non-real-time variable bit rate service is for heavy transmissions that are not real time, such as large file transfers. For this service the base station polls the subscriber often, but not at rigidly-prescribed time intervals. A constant bit rate customer can set a bit in one of its frames requesting a poll in order to send additional (variable bit rate) traffic.

If a station does not respond to a poll k times in a row, the base station puts it into a multicast group and takes away its personal poll. Instead, when the multicast group is polled, any of the stations in it can respond, contending for service. In this way, stations with little traffic do not waste valuable polls.

Finally, best-efforts service is for everything else. No polling is done and the subscriber must contend for bandwidth with other best-efforts subscribers. Requests for bandwidth are done in time slots marked in the upstream map as available for contention. If a request is successful, its success will be noted in the next downstream map. If it is not successful, unsuccessful subscribers have to try again later. To minimize collisions, the Ethernet binary exponential backoff algorithm is used.

The standard defines two forms of bandwidth allocation: per station and per connection. In the former case, the subscriber station aggregates the needs of all the users in the building and makes collective requests for them. When it is granted bandwidth, it doles out that bandwidth to its users as it sees fit. In the latter case, the base station manages each connection directly.

4.5.5 The 802.16 Frame Structure

All MAC frames begin with a generic header. The header is followed by an optional payload and an optional checksum (CRC), as illustrated in Fig. 4-34. The payload is not needed in control frames, for example, those requesting channel slots. The checksum is (surprisingly) also optional due to the error correction in the physical layer and the fact that no attempt is ever made to retransmit real-time frames. If no retransmissions will be attempted, why even bother with a checksum?

Figure 4-34. (a) A generic frame. (b) A bandwidth request frame.

A quick rundown of the header fields of Fig. 4-34(a) is as follows. The EC bit tells whether the payload is encrypted. The Type field identifies the frame type, mostly telling whether packing and fragmentation are present. The CI field indicates the presence or absence of the final checksum. The EK field tells which of the encryption keys is being used (if any). The Length field gives the complete length of the frame, including the header. The Connection identifier tells which connection this frame belongs to. Finally, the HeaderCRC field is a checksum over the header only, using the polynomial x⁸ + x² + x + 1.

A second header type, for frames that request bandwidth, is shown in Fig. 4-34(b). It starts with a 1 bit instead of a 0 bit and is similar to the generic header except that the second and third bytes form a 16-bit number telling how much bandwidth is needed to carry the specified number of bytes. Bandwidth request frames do not carry a payload or full-frame CRC.

A great deal more could be said about 802.16, but this is not the place to say it. For more information, please consult the standard itself.